Fuel cell stack comprising a counterflowing cooling system and a plurality of coolant-collecting ducts located parallel to the axis of the stack

ABSTRACT

A fuel cell stack comprising several superimposed polymer electrolyte membrane fuel cells. A bipolar plate that is provided with longitudinal ducts supplying hydrogen and transverse ducts which supply oxygen and are used for cooling is arranged between adjacent membrane-electrode units. A current flowing in opposite directions through adjacent ducts of the same fuel cell can be created within the air ducts via collecting ducts that are provided at the outflow end for every other cooling duct if the flow is fed to the stack from both sides, resulting in a very homogeneous temperature distribution with the cell or stack.

FIELD OF THE INVENTION

The invention relates to a fuel cell stack according to the featuresspecified in the introductory part of claim 1.

BACKGROUND OF THE INVENTION

Such fuel cell stacks are constructed of fuel cells of thepolymer-electrolyte-membrane construction type and consist of severalcells arranged into a stack. The basic construction of such cells isknown per se, and in this context DE 195 44 323 A1 and DE 199 38 589 A1are referred to.

Fuel cell stacks constructed of such fuel cells are likewise counted asbelonging to the state of the art. A fluid-cooled fuel cell stock with5.5 kW power is offered on the part of Proton Motor GmbH under the partdescription HZ40. With this stock, the supply of fuel on the one handand the supply of oxygen in the form of air supply on the other hand areeffected via central connections, and the distribution within the stackis effected via channel systems. In order to lead away heat which arisesduring operation, a fluid cooling is provided which likewise functionsvia central connections and a channel system led within the bipolarplates.

A problem with such fuel cell stacks is often the removal of thereaction heat arising on account of the catalytic process, which iseither to be led away via the supplied air oxygen or however via aseparate, for example also fluid-leading cooling system. On the one handa fuel cell on operation should have a temperature which is as high aspossible in order to operate with a good efficiency, but on the otherhand the temperature must not be so large that the water stored in thepolymer electrolyte membrane evaporates, since the proton conductivityof the membrane reduces with a falling water content. Therefore anoperating temperature for example of 60° C. to 90° C. is desirable,depending on the applied membrane. This temperature should be asconstant as possible over the surface of a fuel cell as well as withinthe stack so that where possible all fuel cells operate with a highefficiency over their complete surface.

In particular with fuel cell stacks of a small or middle power, afurther problem may occur if, due to unfavorable channel cross sectionsand channel lengths, one must provide a relatively high pressure inorder to lead the coolant through the channel system. The power ofauxiliary units required for this regularly reduces the efficiency.

BREIF SUMMARY OF THE INVENTION

Against this background it is the object of the present invention todesign a fuel cell stack of the initially mentioned type such that an asuniform as possible temperature distribution within the individual fuelcells and within the fuel cell stack is given, with an as low aspossible flow resistance.

According to the invention, the features specified in claim 1 achievethis object. Advantageous formations of the invention are specified inthe dependent claims, the subsequent description as well as thedrawings.

The basic concept of the present invention to lead the cooling fluidwithin each fuel cell of the fuel cell stack such that the through-flowdirection of adjacent channels of the same fuel cell is opposite to oneanother. Since the channels are open at both sides, the inflow is alwayseffected in a parallel manner, which means that the channels are notflowed through in series successively. A very uniform temperaturedistribution within the fuel cell and thus also within the fuel cellstack is achieved by way of this, wherein the flow resistance, inparticular with a suitable connection of the channels as will bedescribed in the following, is comparatively low.

The design of the routing of the channels within a fuel cell for airoxygen in a serpentine or meandering manner is counted as belonging tothe state of the art. With such a design, the through-flow direction ofadjacent channels, although likewise being in opposite directions, withthis however the channels do not lie parallel in the inflow directionbut connected successively, which thermally as well as fluidically issomewhat unfavorable since on the one hand the removal of heat close tothe end at the outflow side as a rule is inadequate and on the otherhand a considerable pressure is to be mustered for the through-flowwhich likewise worsens the efficiency.

The parallel inflow (through-flow) of the channels which are arrangedparallel to one another and are open at both sides, in a manner suchthat the through-flow direction of adjacent fuel channels of the samefuel cells is opposite to one another, in contrast permits a goodcooling with a low flow resistance, which leads to a more uniform heatdistribution within the fuel cell and thus also of the fuel cell stack.

In order not to have to individually route the channels of theindividual fuel cells, but to be able to connect them with littleexpense with regard to manufacturing technology, it is useful to connectthe inflow and outflow sides of channels lying above one another, of thefuel cells arranged into a stack, in a common collector channel whichpreferably runs parallel to the axis of the stack in order thus tocreate a short and thus low-resistance conduit connection.

It is particularly favorable if several collector channels are arrangedparallel to one another and on both sides of the stack so thatpreferably all channels at the inflow side or outflow run into acollector channel. One may realize the inventive flow arrangement in amanner which is simple with regard to design and which is fluidicallyfavorable by way of such collector channels arranged at the end face atthe end of the fuel cells or of the fuel cell stack.

The channels may exclusively or not exclusively serve for the cooling,depending on the energy density in the fuel cell stack. If the channelsexclusively serve as cooling channels, then a fluid which is independentof the remaining function, thus a gas or liquid may be led through thechannels. The quantity of heat which may be removed is comparativelyhigh, particularly with the use of a liquid.

A design with which the cooling channels simultaneously serve for thesupply of oxygen to the fuel cells, and are designed as channels whichare open towards the cathode of the respective membrane electrodeassembly is particularly favorable. Such an arrangement is particularlyfavorable since then a much less complicated oxygen supply may beeffected by way of the supply of surrounding air which where appropriatemay be purified. With such an arrangement one also simultaneouslyachieves an improved oxygen supply of the fuel cell stock with anincreasing removal of heat, which is advantageous. Moreover then therequired energy expense for the energy which is consumed for thethrough-flow for the purpose of cooling is usually lower than with aseparate network of cooling channels.

In the same manner, the channel routing on the anode side may be alsodesigned for the supply of fuel, i.e. then, by way of the fuel, one mayachieve an additional cooling of the respective cell on the anode sidetoo. Where appropriate, as initially described, one may also provide aseparate cooling channel system additionally to an anode-side and/orcathode-side cooling. The inventive channel routing for fuel-leading oroxygen-leading channels, apart from a uniform temperature distributionwithin the fuel cell or the cell stack furthermore has the significantadvantage that the reactands are introduced distributed over the surfaceof the cell in a particularly uniform manner, which leads to a uniformcharging and thus also burdening of the cell.

The cooling channels preferably have a clear width of less than 3 mm,preferably of about 2 mm. Such an arrangement is particularlyadvantageous if the cooling channels also serve for the supply of thestack with air oxygen, since then the abutment contact surfaces of thecarbon layer in which these channels open towards the cathode areusually provided, are designed such that an adequate pressing pressureon the proton exchange membrane is given, so that the membrane iseffective over as much of its surface as possible. On the other hand theabove-mentioned dimensioning ensures that the through-flow of thechannels is ensured with comparatively small flow losses, i.e. with theprovision of only a small excess pressure. At the same time the coolingchannels should usefully have a length between 20 mm and 200 mm. It isto be understood that the clear width of the channels may be smaller,the shorter are the channels and vice versa.

The collector channels by way of which the cooling channels may besupplied at the inflow and outflow side, may be designed in a simplemanner by way of providing suitable recesses in the fuel cell stock atthe edges. These recesses are thus provided in all the layers of thefuel cells which cover this edge region, and thus of the fuel cellstack, preferably in the inactive edge region, so that collectorchannels are formed arranged parallel to the axis of the stack afterassembly of the stock.

If, as is counted as belonging to the state of the art, the bipolarplates of the fuel cells are incorporated in an elastic edge whichsimultaneously forms the lateral sealing of the respective fuel cell tothe outside, then the collector channels may be formed by recesses inthe sealing edges lying above one another. With regard to design,therefore with the exception of the recesses, no special provisions needto be made for creating these collector channels.

Preferably the coolant is supplied with an excess pressure of 0.1 to 10bar or is led away with a corresponding vacuum. Such a pressure may beproduced by blowers, as they are applied for example in semiconductortechnology, e.g. CPU blowers which require little supply energy. Radialfilters which function in a comparatively energy-efficient manner mayeven produce the above-mentioned pressure range.

DESCRIPTION OF THE DRAWINGS

The invention is described in more detail by way of one embodimentexample represented in the drawings. There are shown in:

FIG. 1 in a greatly schematized representation, a fuel cell stockaccording to the invention, with collector channels at the outflow sideand in a greatly schematized representation, a section through thecooling channel system of a fuel cell transverse to the axis of the fuelcell stack according to FIG. 1, along the section line II-II in FIG. 1

DETAILED DESCRIPTION OF THE INVENTION

The fuel cell stack 1 schematically shown in FIG. 1 is constructed in amanner known per se, of a multitude, here six fuel cells 2 which arearranged above one another and are clamped between end plates 3. Eachfuel cell 2 consists of a membrane electrode assembly which is formed byc film 4 in the form of a polymer electrolyte membrane, an anode 5 lyingthereon as well as a cathode 6 lying on the other side. A bipolar plate7 is arranged between adjacent membrane electrode assemblies 4, 5, 6,which is electrically conductive and is formed essentially of carbon.

Each bipolar plate 7 on its side facing the cathode 6 comprisestransverse channels 8 which are open towards both ends, are arrangedparallel to one another and extend transversely to the stack axis 9 aswell as to the longitudinal channels 10 which are likewise providedwithin the bipolar plate 7 and are open towards the anode 5. Thelongitudinal channels 10 serve for the supply of fuel, in particularhydrogen, to the cells. They are formed by grooves on the upper side ofeach bipolar plate 7, said grooves being one towards the anode andrectangular in cross section. The transverse channels 8 in contrastserve the supply of oxygen to the fuel cells 2 as well as for theremoval of heat, thus for cooling. The supply of oxygen as well as thecooling is effected by way of an airflow which is produced by way of ablower and which, with the fuel cell stack 1 represented in FIG. 1, ispresent on the left side as well as the right side of the stack by wayof a suitable channel routing (not shown).

Since the routing of the air (arrows), as described initially, within afuel cell 2 is designed such that the flow runs in opposite directionsin adjacent transverse channels 8 of each fuel cell 2, the outlets ofthe transverse channels 8 of the fuel cells 2 arranged above one anotherare connected to one another in a conducting manner by way of collectorchannels 11 arranged parallel to the stack axis 9, as is evident fromFIG. 1. The collector channels 11 which in FIG. 1 are represented bycomponents which are U-shaped in cross section, may be designed invarious manners, as has been explained initially. They are designed andarranged such that they connect the outflow sides of the ends of thetransverse channels 8 of all fuel cells 2 in a conducting manner, saidends lying above one another in the axial direction 9, but do not affectthe inflow sides in each case of channels 8 adjacent to the right andleft. The components therefore are designed and arranged such that withan inflow of air of the fuel cell stack from the left and right sideseeing in the Figure, in each fuel cell 2 in the transverse channels 8,a flow sets in as is represented by way of example in FIG. 2 by way ofthe cross section through the air channels 8 of a fuel cell 2 (by way ofarrow representation).

With the shown embodiment example, the channel connection by way of thecollector channels 11 is effected in each case on the outflow side. Itmay however also be effected at the inflow side which would means areversal of all flow directions in the figures.

The collector channels represented schematically in FIGS. 1 and 2 as arule are formed on construction of the fuel cell stack 1 in thatsuitable recesses are formed in the edge region of the components forexample 4, 5, 6, 7 or their enclosure at the edge. The collectorchannels 11 at the some time are to be designed such that an as low aspossible flow resistance results.

In order to keep down the flow resistance also within the fuel cells, inparticular for the supply of air, the transverse channels 8 are suitablydimensioned with regard to cross section and length. In the presentembodiment example, the transverse channels 8 have a clear width of 2 mmwith a length of 100 mm. In this manner one may ensure a goodthrough-flow of the fuel cell stack 1 even with only a slight excesspressure. In the present example, with a suitable channel routing to theend-faces of the fuel cell stack 1, a small radial blower or CPU bloweris sufficient in order to adequately supply the fuel cell stock 1 withoxygen as well as with cooling air.

The above-described arrangement as a rule ensures a very uniformtemperature distribution within the fuel cell stack 1. If the cooling ofsuch an arrangement is not sufficient or a separate cooling system is tobe arranged for other reasons, then this may be effected by way of asuitable arrangement of cooling channels, for example in the bipolarplate 7 between the transverse channels 8 and the longitudinal channels10 with a suitable channel routing via collector channels 11. It is tobe understood that in this case the conducting connection for thechannels 8 leading the oxygen needs to be provided separately for thechannels 10 leading the fuel.

1-11. (canceled)
 12. A fuel cell stack, comprising: a plurality of fuelcells arranged in a stack, each of said fuel cells being of polymerelectrolyte membrane construction and comprising a membrane electrodeassembly, each of the fuel cells defining a plurality of essentiallyparallel channels for conducting cooling fluid between adjacent membraneelectrode assemblies, each of said channels having two open ends,wherein a direction of flow of one of said channels is opposite to thedirection of flow of adjacent ones of said channels in said each of saidfuel cells.
 13. The fuel cell stack of claim 12, wherein each of saidchannels has an inflow side and an outflow side, said fuel cell stackfurther comprising a common collector channel, wherein one of inflowsides and outflow sides of said channels that are arranged one above theother are connected in said common conductor channel.
 14. The fuel cellstack of claim 13, further comprising a plurality of common collectorchannels arranged in parallel on two sides of said fuel cell such thateach of the one of inflow sides and outflow sides of said channels runinto one of said plural common collector channels.
 15. The fuel cellstack of claim 12, wherein said plural channels are arranged exclusivelyfor cooling said fuel cells, said plural channels conducting one of agas and a fluid.
 16. The fuel cell stack of claim 12, wherein each ofsaid membrane electrode assemblies comprises an anode electrode and acathode electrode, wherein said channels are open toward said cathodeelectrode and conduct an oxygen supply toward said cathode electrodes.17. The fuel cell stack of claim 12, wherein each of said membraneelectrode assemblies comprises an anode electrode and a cathodeelectrode, wherein said channels are open toward said anode electrodeand conduct a fuel supply toward said anode electrodes.
 18. The fuelcell stack of claim 12, wherein said channels have a width of less than3 mm.
 19. The fuel cell stack of claim 18, wherein said channels have alength in the range 20 mm to 200 mm.
 20. The fuel cell stack of claim13, wherein said stack comprises a recess at an end thereof forming saidcommon collector channel.
 21. The fuel cell stack of claim 13, furthercomprising an elastic sealing edge surrounding a bipolar plate of saideach of said fuel cells and arranged between adjacent fuel cells, saidcommon collector channel being formed by recesses in said sealing edgeslying above one another.
 22. The fuel cell stack of claim 12, whereinsaid channels are arranged such that an adequate supply of coolant issupplied with an excess pressure of 0.1 to 10 bar.
 23. The fuel cellstack of claim 13, wherein said stack comprises an axis through a centerof each fuel cell and said common collector channel runs parallel tosaid axis of said stack.
 24. The fuel cell stack of claim 18, whereinsaid channels have a width approximately 2 mm.
 25. The fuel stack ofclaim 12, wherein said channels have a length in the range 20 mm to 200mm.
 26. The fuel stack of claim 12, wherein said channels are arrangedsuch that an adequate supply of coolant is drawn by a vacuum or negativepressure of 0.1 to 10 bar.
 27. The fuel cell stack of claim 12, whereinsaid common collector channel is formed by an enclosure along an edge ofsaid each one of said fuel cells.